Slurry hydrocracking (SHC) is a slurried catalytic process used to crack residue feeds to gas oils and fuels. SHC is used for the primary upgrading of heavy hydrocarbon feed stocks obtained from the distillation of crude oil, including hydrocarbon residues or its mixture with gas oils from atmospheric distillation tower bottoms or vacuum distillation tower bottoms. Alternative sources of heavy hydrocarbon feed stocks include solvent deasphalted pitch or visbroken residues. In slurry hydrocracking, these liquid feed stocks are mixed with hydrogen and solid catalyst particles, e.g., as a particulate metallic compound such as a metal sulfide, to provide a slurry phase. Slurry hydrocracked effluent exits the slurry hydrocracking reactor at very high temperatures around 400° C. (752° F.) to 500° C. (932° F.). Representative SHC processes are described, for example, in U.S. Pat. Nos. 5,755,955 and 5,474,977.
During an SHC reaction, it is important to minimize coking. It has been shown by the model of Pfeiffer and Saal, PHYS. CHEM. 44, 139 (1940), that asphaltenes are surrounded by a layer of resins, or polar aromatics which stabilize them in colloidal suspension. In the absence of polar aromatics, or if polar aromatics are diluted by paraffinic molecules or are converted to lighter paraffinic and aromatic materials, these asphaltenes can self-associate to form aggregates, thus forming larger molecules in a separate liquid phase, generate mesophase and form coke.
A fraction of SHC product that is not dissolved by toluene include catalyst and toluene insoluble organic residue (TIOR). TIOR includes coke and mesophase and is heavier in density and less soluble than asphaltenes which are soluble in toluene but not heptane. Mesophase formation is a critical reaction constraint in slurry hydrocracking reactions. Mesophase is a carbonaceous, liquid-crystal material defined as anisotropic particles present in pitch boiling above 524° C. The presence of mesophase can serve as a warning that excessive coke formation is likely to occur under prevailing conditions.
Coking can be minimized by the use of an additive or controlled by lowering reaction temperature. However, at lower temperature, more reactor volume is required to convert feeds with lower reactivity. Adding a polar aromatic oil to the feedstock of a SHC reactor is effective in reducing the coke formation as described in U.S. Pat. No. 5,755,955. Furthermore, U.S. Pat. No. 6,004,453 describes such SHC processing with recycle of both heavy gas oil and unconverted pitch to enable the operation of the unit at a higher conversion, thus facilitating residue upgrading.
One difficulty with SHC processes is the increased risk of reactor fouling when operating at higher pitch conversion. Pitch recycle can be used to reduce the potential for reactor fouling and is necessary to achieve high pitch conversion.
In a SHC process, separation of pitch from vacuum gas oil (VGO) is typically achieved by a vacuum fractionation column that cooperates with an upstream series of separators, stripper(s) and an atmospheric fractionation column. Atmospheric and vacuum fractionation columns provide streams with specified boiling point ranges that are transported to downstream upgrading units. Other recovered products include naphtha, kerosene and diesel.
SHC products typically require additional heating before delivery to a vacuum fractionation column. To achieve low VGO concentration and high pitch concentration in the vacuum bottom stream, the fractionator feed stream and column vaporization zones require high temperature to make up for losses in vacuum pressure encountered while passing through vacuum packing materials along the height of the column. Packing materials are added to achieve efficient separation of product streams. High temperature increases the risk of severe fouling. Otherwise, poor separation can result in high concentrations of VGO product or even light products in the vacuum column bottoms.
In petroleum processing, a mixture of two or more organic compounds can form second liquid phase or semi-solid deposit upon mixing due to their thermodynamic properties. Such compounds are termed “incompatible”. A solvent is incompatible with a hydrocarbon stream when mixing of the two creates a second liquid phase or semi-solid deposit. Such an incompatible solvent is considered a poor solvent for that hydrocarbon compound. A solvent is considered to be a “poor solvent’ relative to a hydrocarbon stream due to a number of factors. The solubility parameters of the hydrocarbon stream and the solvent stream play a key role. A general guidance is the lower the solubility parameter of a solvent, the higher tendency for this solvent to behave like a poor solvent upon mixing with a hydrocarbon stream. Poor solvents for slurry hydrocracking products can be good solvents for solvent deasphalting because compounds present in slurry hydrocracked product are more dealkylated than naturally occurring hydrocarbon compounds in oil streams. Intrinsically, the physical properties of all participating compounds of a mixture system determines the compatibility behavior; e.g. dispersion energy, molar volume, carbon number, hydrocarbon type and its polarity. A common procedure measures stability of an oil stream through asphaltene flocculation or precipitation by titrating it with a poor solvent compound. ASTM 6703 and Heithaus, JOURNAL OF THE INSTITUTE OF PETROLEUM, 45, 48 (1962) have described similar techniques commonly used in petroleum processing industry.
There is a continuing need, therefore, for improved processes and apparatuses for upgrading residue feed stocks in slurry hydrocracking and in suppression of mesophase production. Improved apparatuses and processes for recovering products and separation of pitch from VGO are needed for SHC recovery processes.